Facile Preparation of Robust Superhydrophobic/Superoleophilic TiO2-Decorated Polyvinyl Alcohol Sponge for Efficient Oil/Water Separation.

Oily wastewater and oil spills pose a threat to the environment and human health, and porous sponge materials are highly desired for oil/water separation. Herein, we design a new superhydrophobic/superoleophilic TiO2-decorated polyvinyl alcohol (PVA) sponge material for efficient oil/water separation. The TiO2-PVA sponge is obtained by firmly anchoring TiO2 nanoparticles onto the skeleton surface of pristine PVA sponge via the cross-linking reactions between TiO2 nanoparticles and H3BO3 and KH550, followed by the chemical modification of 1H,1H,2H,2H-perfluorodecyltrichlorosilane. The as-prepared TiO2-PVA sponge shows a high water contact angle of 157° (a sliding angle of 5.5°) and an oil contact angle of ∼0°, showing excellent superhydrophobicity and superoleophilicity. The TiO2-PVA sponge exhibits excellent chemical stability, thermal stability, and mechanical durability in terms of immersing it in the corrosive solutions and solvents, boiling it in water, and the sandpaper abrasion test. Moreover, the as-prepared TiO2-PVA sponge possesses excellent absorption capacity of oils or organic solvents ranging from 4.3 to 13.6 times its own weight. More importantly, the as-prepared TiO2-PVA sponge can separate carbon tetrachloride from the oil-water mixture with a separation efficiency of 97.8% with the aid of gravity and maintains a separation efficiency of 96.5% even after 15 cyclic oil/water separation processes. Therefore, the rationally designed superhydrophobic/superoleophilic TiO2-PVA sponge shows great potential in practical applications of dealing with oily wastewater and oil spills.

Introduction

Modern industries and frequent oil spill incidents have produced a large quantities of oily wastewater, which causes serious pollution in the environment.[1] The toxic chemicals in oily wastewater seriously threaten the survival of aquatic organisms, and some toxic substances enter the human body through the food chain and significantly influence human health.[2] To mitigate the threat of oily wastewater, traditional approaches have been utilized to purify oily wastewater, including controlled burning, air flotation, chemical dispersion, and bioremediation.[3,4] However, these methods suffer from secondary pollution, high cost, and low efficiency.[5,6] Thus, it is necessary to develop low-cost, environmentally friendly, stable, and efficient approaches or materials for the treatment of oily wastewater.

Bio-inspired from lotus leaves and insects, superhydrophobic materials (contact angle >150° and contact angle hysteresis <10°) have been widely investigated for many promising applications, such as anti-icing,[7−10] anti-corrosion,[11] sensor,[12] self-cleaning,[13,14] drag reduction,[15] and oil/water separation.[16] In recent years, various superhydrophobic materials have been developed and utilized for efficient oil/water separation, including meshes,[17] sponges,[18] textiles,[19] and filter papers.[20] Among them, superhydrophobic porous materials show the most stable and efficient properties for oil/water separation because of their large surface area, light weight, high stability, and high porosity.[21,22] Superhydrophobic porous materials usually possess three-dimensional (3D) structures and overcome the drawback of pore plugging present in the two-dimensional oil/water separation materials. To date, a series of 3D superhydrophobic porous materials with oil/water separation function have been achieved,[21] such as polyurethane sponges,[16] melamine sponges,[23] aerogels,[24,25] membranes,[26−28] fabrics,[29] and foam rubbers.[30]

Polyvinyl alcohol (PVA) sponge is a 3D porous material with low cost, high biocompatibility, and high porosity.[31] It is often used in the fields of tissue engineering,[32] adsorption materials,[33,34] and catalysts,[35] while there are only a few studies on superhydrophobic PVA-based sponges for oil/water separation.[36−39] For example, Chen et al. prepared a superhydrophobic PVA sponge by using a facile and environment-friendly route, and the as-prepared PVA sponge showed long stability for continuous oil/water separation.[36] Sha et al. reported that 3D superhydrophilic PVA-based composite sponges with high porosity can be used as efficient emulsion separation materials.[37,38] Wang et al. obtained a series of silane functionalized PVA formaldehyde sponges, which exhibited oil absorption capacities ranging from 4.0 to 14.0 g·g–1 in different solvents.[39] Wang et al. fabricated superhydrophobic PVA/Na2SiO3 porous materials and realized oil adsorption capacities ranging from 1.8 to 7.0 g·g–1 for different oil liquids.[40] Generally, pristine PVA sponge contains rich hydroxyl groups, which can help active sites to react with nanoparticles and thus obtain hierarchical structures on the skeletons of the PVA sponge. Besides, the durability of PVA-based sponges is also important for the long-term use of oil/water separation.[41] Shang et al. reported a superhydrophobic PDA-decorated melamine-formaldehyde sponge that still shows good superhydrophobicity after abrasion tests by a piece of sandpaper.[42] Wang et al. constructed superhydrophobic porous sponges by a mussel-inspired one-step strategy and evaluated the mechanical durability of the sponges by repeating the cycles of compression and release.[43] In short, superhydrophobic porous PVA sponges are promising porous materials and can be utilized for efficient oil/water separation.[44]

Herein, a new superhydrophobic/superoleophilic TiO2-decorated PVA sponge material has been prepared for the first time for efficient oil/water separation, showing excellent chemical stability in harsh environments, thermal stability in boiling water, and mechanical durability after sandpaper abrasion test. Because of abundant hydroxyl groups of the PVA sponge, the as-prepared TiO2–PVA sponge is successfully obtained by firmly anchoring TiO2 nanoparticles onto the skeleton surface of pristine PVA sponge and chemically modifying with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), exhibiting a high water contact angle (WCA) of 157° (sliding angle of 5.5°) and an oil contact angle of ∼0°. More importantly, the as-prepared TiO2–PVA sponge possesses excellent absorption capacity of oils (or organic solvents) ranging from 4.3 to 13.6 times its own weight. The TiO2–PVA sponge also exhibits a separation efficiency of 97.8% for separating carbon tetrachloride from the oil–water mixture and maintains a separation efficiency of 96.5% even after 15 cyclic oil/water separation processes. Thus, the rational design of superhydrophobic/superoleophilic TiO2–PVA sponge provides new insights into the practical applications of dealing with oily wastewater and oil spills.

Results and Discussion

Preparation and Surface Morphology of the TiO2–PVA Sponge

The schematic overview of the preparation of superhydrophobic/superoleophilic TiO2–PVA sponge is illustrated in Figure . The whole process of preparing the TiO2–PVA sponge mainly contains three steps. First, the H3BO3 and TiO2 nanoparticles are added to the NaOH solution at pH = 10, and the reaction is carried out at 85 °C for 2 h. During this step, strong hydrogen bonds can be formed due to the rich hydroxyl groups of TiO2 and three hydroxyl groups of H3BO3. Next, the silane coupling agent (KH550) and the TiO2–PVA sponge are added to the resulting solution, and the reaction is continued under magnetic stirring.[45,46] The silane coupling agent usually acts as a bridge between the H3BO3 cross-linked TiO2 nanoparticles and the pristine PVA sponge. The formation of N–B bonds between H3BO3 and KH550 can improve the cross-linking efficiency of TiO2 nanoparticles onto the polymer chains.[47,48] In this process, the dehydration and condensation reaction between the hydroxyl groups Ti–OH and hydroxyl groups of the PVA sponge can enhance the formation of the TiO2/PVA nanocomposite.[49,50] Besides, the cross-linking reaction of KH550 and H3BO3 can make TiO2 nanoparticles firmly anchored on the skeletons of the pristine PVA sponge and thus construct rough micro-nanoscale structures. Finally, the superhydrophobic/superoleophilic TiO2–PVA sponge is obtained by lowering surface energy with an FDTS solution.

Figure 1

Schematic overview of preparing the TiO2–PVA sponge.

The morphologies of the pristine PVA sponge and the superhydrophobic/superoleophilic TiO2–PVA sponge are shown in Figure . The inherent 3D porous structures of the pristine PVA sponge with macro-scale pores are shown in Figure a. To evaluate the morphologies of the skeletons of pristine PVA sponge, a higher magnified image is displayed in Figure b, and it can be found that the skeleton of pristine PVA sponge is very smooth. After being modified with a low surface energy material (FDTS), rough porous structures of TiO2–PVA sponge are formed, as illustrated in Figure c–f. The TiO2–PVA sponge maintains the 3D porous structures of the pristine PVA sponge after the reaction and modification processes (Figure c), and the skeleton of the TiO2–PVA sponge becomes rough. As shown in the highlighted areas in Figure d, TiO2 nanoparticles are uniformly covered on the skeletons of the TiO2–PVA sponge (Figure e,f), which is due to the deposition of TiO2 nanoparticles cross-linked with H3BO3 and KH550.[46,47] Before the deposition process, the average size of TiO2 nanoparticle dispersion in the presence of H3BO3 is about 48 nm, characterized using dynamic light scattering (Figure S2). Due to the cross-linking of KH550, TiO2 dispersion becomes aggregated and finally forms TiO2-based micro-structures (∼0.22 μm) on the skeletons of the PVA sponge, as shown in Figure f. Thus, the obtained micro-nanoscale structures of the TiO2–PVA sponge and the chemical modification of low surface energy give the TiO2–PVA sponge superhydrophobicity and excellent water repellency.

Figure 2

SEM images of the (a,b) pristine sponge and (c–f) superhydrophobic/superoleophilic TiO2–PVA sponge with different magnifications. (e,f) correspond to the highlighted areas in red and green, respectively.

Chemical Composition of the TiO2–PVA Sponge

In order to confirm the chemical composition of the TiO2–PVA sponge, energy-dispersive spectrometry (EDS) mapping, Fourier transform infrared (FTIR) spectra, and X-ray diffraction (XRD) pattern are analyzed in Figure . The EDS mapping of the TiO2–PVA sponge is illustrated in Figure a. Compared with the elements of the pristine PVA sponge (i.e., C and O), the scan areas are almost covered by the elements of C, O, and B (Figure b–d), which means that H3BO3 has reacted and has been uniformly distributed on the skeletons of the pristine PVA sponge. Besides, it can be found that the EDS mapping (Figure b–f) and the EDS spectra (Figure g) of the TiO2–PVA sponge contain the elements of F and Ti, indicating the successful deposition of TiO2 nanoparticles and low surface modification of FDTS on the pristine PVA sponge.

Figure 3

EDS mapping (a–f) and EDS spectra (g) of the TiO2–PVA sponge; FTIR spectra (h) and XRD pattern (i) of the pristine PVA sponge and the TiO2–PVA sponge.

The FTIR spectra of the pristine PVA sponge and the TiO2–PVA sponge are shown in Figure h. As for the FTIR curve (black line) of the pristine PVA sponge, the typical peak at 3410 cm–1 is ascribed to the characteristic peak of rich hydroxyl groups of the PVA sponge. The observed peaks at 2860–2942 and 1002 cm–1 are due to the broad C–H alkyl stretching bands and the C–O stretching bands of the PVA sponge.[51] Another characteristic peak at 1658 cm–1 refers to the C=O band, which is possible due to the incomplete hydrolysis of poly(vinyl acetate).[52,53] The peaks at 1408 and 791 cm–1 are from the C–H bending and the stretching vibration of −CH2.[54−56] Furthermore, the FTIR curve of the TiO2–PVA sponge (red line) is also illustrated in Figure h. It can be found that the typical peak of hydroxyl groups of the TiO2–PVA sponge significantly decreases because of superhydrophobicity. The peak at 476 cm–1 belongs to the stretching vibration of Ti–O,[49,57] indicating the deposition of TiO2 nanoparticles on the skeleton of the PVA sponge. The absorption peaks at 1461 cm–1 are assigned to the B–O–B group,[46] and it means that H3BO3 has been successfully grafted onto the skeleton of the PVA sponge. The Ti–O–B band appears at 1398 cm–1, showing that TiO2 nanoparticles have successfully reacted with H3BO3.[58,59] Furthermore, the peaks at 1346 and 1131 cm–1 refer to the absorption band of N–B[47] and the stretching vibration of Si–O–C,[60,61] verifying that KH550 has reacted with the PVA sponge and TiO2 nanoparticles have been firmly anchored onto the skeletons of the PVA sponge. In addition, the observed peak at 1242 cm–1 is due to the stretching vibration of the C–F group,[61,62] revealing the chemical modification of FDTS on the TiO2–PVA sponge. Thus, the preparation of the TiO2–PVA sponge has been demonstrated by the FTIR spectra when compared with that of the pristine PVA sponge.

The XRD patterns of the pristine PVA sponge and the TiO2–PVA sponge are shown in Figure i. The XRD pattern of pristine PVA sponge displays a typical diffraction peak at 2θ = 19.5° and demonstrates the amorphous nature of the pristine PVA sponge.[63,64] After decorating with TiO2, the XRD pattern of the TiO2–PVA sponge shows peaks at 28.2, 36.8, 41.8, 44.8, 54.9, 57.4, 63.4, 65.0, and 69.9°, which is in agreement with the crystal form of rutile TiO2 (PDF Card 88-1174).[65] In short, the XRD pattern of the TiO2–PVA sponge shows the successful deposition of TiO2 nanoparticles on the skeletons of the pristine PVA sponge.

Wettability of the TiO2–PVA Sponge

The surface wetting properties of the pristine PVA sponge and the TiO2–PVA sponge are shown in Figure . It can be seen that the water and oil droplets can be immediately absorbed into the pristine PVA sponge when they contact the pristine PVA sponge with superhydrophilicity and superoleophilicity (Figure a). For comparison, the modified TiO2–PVA sponge exhibits different wettability. It can be seen from Figure b that the TiO2–PVA sponge retains its superoleophilicity, while it shows superhydrophobicity for a water droplet (dyed with Brilliant Green). The water droplet shows a spherical shape on the TiO2–PVA sponge, and its WCA is 157°. Interestingly, when the TiO2–PVA sponge is completely immersed in water by an external force, a mirror-like reflection phenomenon can be observed due to the trapped air on the TiO2–PVA sponge surface (Figure c). Moreover, the TiO2–PVA sponge also exhibits ultra-low adhesion to water droplets. As shown in Figure d1–d4, the water droplet can easily slide off from the surface of the TiO2–PVA sponge, showing a strong water repellent property. A water droplet can easily be detached from the surface of the TiO2–PVA sponge when it comes in contact with the surface of the TiO2–PVA sponge (Figure e1–e4). In the above discussion, the superoleophilicity and superhydrophobicity of the TiO2–PVA sponge have been demonstrated.

Figure 4

Images of the water droplet (dyed with Brilliant Green) and oil droplet (dyed with Sudan I) on the surface of the pristine PVA sponge (a) and the TiO2–PVA sponge (b). (c) Mirror-like phenomenon of the TiO2–PVA sponge when immersed in water. (d1–d4) Photographs of a water droplet sliding off from the TO2–PVA sponge. (e1–e4) TiO2–PVA sponge shows low adhesion for a water droplet.

Chemical Stability and Mechanical Durability of the TiO2–PVA Sponge

Chemical stability is an important factor to characterize the TiO2–PVA sponge for its real practical application. Herein, the chemical stability of the TiO2–PVA sponge is tested by immersion tests in 1 M NaCl solution, 1 M HCl solution, and 1 M NaOH solution for different immersion times (0–25 h) (Figure a). The WCAs on the TiO2–PVA sponge show a gradual continuous downward trend as the immersion time increases in different corrosive solutions. It can be seen from Figure a that the WCA (153°) on the TiO2–PVA sponge shows a very small decrease after being immersed in 1 M NaCl solution for 25 h, indicating good chemical stability of the TiO2–PVA sponge in 1 M NaCl solution. However, when immersed in 1 M HCl and 1 M NaOH solutions for 20 h, the WCA on the TiO2–PVA sponge reduces to around 150° and continues to decrease with the increase in immersion time in the corrosive solutions. In the above harsh environments, the chemical stability of the TiO2–PVA sponge cannot remain for a long time (i.e., longer than 20 h), and thus, the corresponding mechanical durability of the TiO2–PVA sponge can gradually decrease as time elapses. Generally, the TiO2–PVA sponge possesses excellent chemical stability for solutions with different pH values. Besides, the solvent and oil resistance of the TiO2–PVA sponge is evaluated by immersing the TiO2–PVA sponge in ethanol, n-hexane, and tetrachloromethane (CCl4) for 24 h (Figure b). It can be found that the WCAs are still above 155°, and the TiO2–PVA sponge displays excellent solvent and oil resistance properties.

Figure 5

WCAs of the TiO2–PVA sponge after being immersed in (a) 1 M NaCl solution, 1 M HCl solution, and 1 M NaCl solution for different times and (b) ethanol, n-hexane, and CCl4 for 25 h. WCAs and sliding angles of the TiO2–PVA sponge after (c) being immersed in boiling water for different times and (d) sandpaper abrasion test. (e) Schematic diagram of a sandpaper abrasion test.

In addition, the thermal stability of the TiO2–PVA sponge is investigated by immersing the as-prepared sponge in boiling water. The WCAs and sliding angles on the TiO2–PVA sponge are illustrated in Figure c. It is found that the WCA and the sliding angle on the TiO2–PVA sponge are 156.6 and 5.5°, respectively, at the beginning of the test. After being immersed in boiling water (0–60 min), the WCA drops to ∼150° at 50 min and deceases below 150° at 60 min, and the sliding angle gradually increases to about 11.9° at 60 min, indicating that the TiO2–PVA sponge possesses good thermal stability in boiling water within 60 min.

The mechanical robustness of the TiO2–PVA sponge is characterized by a sandpaper abrasion test, as shown in Figure d,e. The TiO2–PVA sponge is placed under a load of 100 g on the sandpaper (800 mesh) (Figure e) and then is moved with a length of 10 cm for 30 cycles. It can be seen from Figure d that the sliding angle on the TiO2–PVA sponge becomes more than 10° after 20 cycles, and the WCA on the TiO2–PVA sponge reduces to 149° after 25 cycles. This means that the TiO2–PVA sponge gradually loses its superhydrophobicity during the abrasion cycles. Similarly, Parsaie et al. found that the contact angle of the superhydrophobic polyurethane (PU) sponge does not show obvious changes after 10 cycles of abrasion tests.[2] You et al. reported that chitosan–PVA–TiO2-coated copper mesh still maintains its superhydrophobicity after 25 cycles of abrasion tests.[17] Furthermore, ultrasonification is also used to evaluate the durability of the TiO2–PVA sponge in ethanol for different times (i.e., 0–60 min). The results show that the contact angle on the TiO2–PVA sponge remains above 150° (Figure S1), indicating its excellent superhydrophobicity and durability. In short, the as-prepared superhydrophobic/superoleophilic TiO2–PVA sponge exhibits excellent chemical stability, thermal stability, and mechanical durability and thus shows great potential in the application of oil/water separation.

Oil Adsorption Performance of the TiO2–PVA Sponge

The superhydrophobic/superoleophilic TiO2–PVA sponge with high porosity can be used as an ideal material for efficient oil/water separation. Herein, the TiO2–PVA sponge is used as an adsorption material to separate different oils mixed in water through selective absorption. To simulate a practical adsorption process, oils with different densities are utilized for absorption by the superhydrophobic/superoleophilic TiO2–PVA sponge. It can be seen from Figure a1–a4 that the light oil (n-hexane dyed with Sudan I) floats on the surface of water (dyed with Brilliant Green). When the TiO2–PVA sponge is forced to contact with n-hexane, it immediately absorbs n-hexane. Similarly, the TiO2–PVA sponge can also be used to absorb heavy oil (CCl4 dyed with Sudan I) from the bottom of the water, as shown in Figure b1-b4. In the whole absorption process, the TiO2–PVA sponge is not contaminated by the Brilliant Green-dyed water, indicating that the superhydrophobicity of the TiO2–PVA sponge can be maintained during the oil absorption process. Furthermore, obtaining absorbed oils from the TiO2–PVA sponge is also important for the oil/water separation application. The absorbed oils with low viscosity in this study can be easily obtained by drying, while the recovery of oils with high viscosity can be realized by mechanically compressing, washing with anhydrous ethanol, and drying.[66,67]

Figure 6

Images of the TiO2–PVA sponge absorption process of (a1–a4) n-hexane (dyed with Sudan I) and (b1–b4) CCl4 (dyed with Sudan I).

Oil/Water Separation Performance of the TiO2–PVA Sponge

Based on the above good oil absorption performances, the oil absorption capacity of the superhydrophobic/superoleophilic TiO2–PVA sponge is further studied. In order to achieve oil/water separation, an oil/water separation device is designed, as exhibited in Figure a1–a4. When the mixture of oil (CCl4 dyed with Sudan I) and water (dyed with Brilliant Green) is poured into the separation device, water is tightly blocked by the TiO2–PVA sponge sample and oil can easily flow through the TiO2–PVA sponge into the beaker below. These results indicate the excellent superhydrophobicity and superoleophilicity of the TiO2–PVA sponge.[24] As shown in Figure b, the maximum adsorption capacity of the TiO2–PVA sponge is evaluated by the adsorption capacity of oils and solvents [i.e., polyethylene glycol (PEG), CCl4, liquid paraffin, N,N-dimethylformamide, ethanol, edible oil, and n-hexane]. The TiO2–PVA sponge displays excellent oil adsorption capacities ranging from 4.3 to 13.6 times its own weight, as shown in Table , depending on the density and viscosity of the oil and organic solvents and the porosity of the TiO2–PVA sponge.[2,23,24,44] Because of abundant hydroxyl groups, the pristine PVA sponge can be modified to possess superhydrophilicity (or superoleophilicity) such that different kinds of wastes (i.e., ions, oils, and/or organic solvents) can be absorbed (Table ). Compared with PVA-based sponges, other superhydrophobic sponge materials (i.e., PU sponge, melamine sponge, cellulose sponge, and graphene sponge) usually show higher oil adsorption capacities because of different porosities and surface chemistry properties.[68]

Figure 7

(a1–a4) Separation process of the water (dyed with Brilliant Green) mixture with CCl4 (dyed with Sudan I) with the TiO2–PVA sponge; (b) absorption capacity of the TiO2–PVA sponge for different oils and organic solvents; (c) absorption recyclability of the TiO2–PVA sponge for n-hexane and CCl4; (d) variation in separation efficiency of the TiO2–PVA sponge versus the number of oil/water separation cycles. (e) Separation efficiency of the TiO2–PVA sponge for separating CCl4 from different corrosive solution mixtures.

Table 1

Comparison of the Adsorption Capacity and Recyclability of PVA-Based Sponges

sponge	modifying materials	adsorbed materials	adsorption capacity (g/g)	recyclability (times)	ref
PVA sponge	poly(acrylic acid), Prussian Blue	c	4.082 × 10–3		(33)
PVA sponge	Zn/Fe layered double hydroxide	As (V) anions	8.57 × 10–2	5	(34)
PVF sponge	PVA–COOH	water	9.5	10	(37)
PVF sponge	PVA and chitosan (or diatomite, sodium alginate)	water	6.4–14.6	10	(38)
PVF sponge	dodecyltrimethoxysilane	oils, organic solvents	4.0–14.0	10	(39)
PVA sponge	trimethoxy(octadecyl)silane, Na2SiO3	oils, organic solvents	1.8–7.0	10	(40)
PVA sponge	TiO2, KH550, H3BO3, FDTS	oils, organic solvents	4.3–13.6	15	this work

In addition, the recyclability of the superhydrophobic/superoleophilic sponge materials for oil/water separation is also an important parameter for the long-term practical oil/water separation. Thus, n-hexane and CCl4 are utilized to investigate the recyclability of the TiO2–PVA sponge. As presented in Figure c, the absorption capacity of the TiO2–PVA sponge for n-hexane and CCl4 does not show an obvious decrease even after 15 cyclic absorption tests, suggesting that the TiO2–PVA sponge is an ideal material to separate oils from the water mixture. Furthermore, the calculated separation efficiency of the TiO2–PVA sponge separating CCl4 from the water mixture reaches about 97.8%, and the separation efficiency still maintains 96.5% even after 15 cyclic oil/water separations, as shown in the Figure d. Besides, the stability of the TiO2–PVA sponge in corrosive solutions (i.e., 1 M NaOH, 1 M HCl, and 1 M NaCl) is investigated by repeated separation cycles for the mixture of CCl4 and water. The TiO2–PVA sponge exhibits a high separation efficiency (∼97.5%) for corrosive oil/water mixtures (Figure e). In short, the superhydrophobic/superoleophilic TiO2–PVA sponge possesses excellent oil/water separation, recyclability, and chemical resistance properties and has great potential in the practical application of treating oily wastewater and oil spills.

Continuous Separation Capability of the TiO2–PVA Sponge

In real practical applications, the efficiency of oil adsorption and oil/water separation is of great importance to treat frequent oil spills and large quantities of oily wastewater. Thus, a continuous separation device for oil/water separation is designed with the assistance of a vacuum pump, as shown in Figure . The superhydrophobic/superoleophilic TiO2–PVA sponge is attached to a tube connected with the vacuum pump, and the TiO2–PVA sponge is placed at the oil/water interface (Figure a). After turning on the vacuum pump, the Sudan I-dyed n-hexane is continuously pumped into the Büchne flask, as shown in Figure b–d, and n-hexane in the mixed solution can be completely separated into the Büchne flask in a short time. During a separation process, the TiO2–PVA sponge retains superhydrophobicity, and there is no water in the collected n-hexane. As for the continuous oil/water separation, the saturated absorption problem of superhydrophobic/superoleophilic sponge materials can be automatically solved with the aid of the vacuum pump. For example, Chen et al. found that silylated PVA sponge can achieve a total of 7600 times its own weight by the continuous separation.[36] Wang et al. suggested that oils with high viscosity can be treated by continuous separation.[16] Thus, the continuous oil/water separation shows great potential in dealing with large-scale oil spill accidents.

Figure 8

Pictures of vacuum pump-assisted continuous removal of n-hexane (dyed with Sudan I) from the water surface.

Conclusions

In summary, a new superhydrophobic/superoleophilic TiO2–PVA sponge is prepared by firmly anchoring TiO2 nanoparticles onto the skeletons of the pristine PVA sponge via a cross-linking reaction between TiO2 and KH550 and H3BO3. The as-prepared TiO2–PVA sponge shows excellent chemical stability, thermal stability, and abrasion resistance in harsh environments. Due to the porosity and superhydrophobicity/superoleophilicity, the absorption capacities of the TiO2–PVA sponge for various oils and organic solvents range from 4.3 to 13.6 times their own weight. When used in an oil/water separation test, the separation efficiency of the TiO2–PVA sponge reaches ∼97.8%. Besides, the TiO2–PVA sponge also exhibits good recyclability with a separation efficiency of ∼96.5% even after 15 cyclic oil/water separation processes. Furthermore, the continuous oil/water separation of the TiO2–PVA sponge further demonstrates its great potential in practical applications of dealing with oily wastewater and large-scale oil spills. Therefore, the rational design of the superhydrophobic/superoleophilic TiO2–PVA sponge opens an avenue for preparing new porous sponge materials and also provides new insights into efficient oil/water separation.

Experimental Section

Materials

The PVA sponge was purchased from a local supermarket. 3-Aminopropyltriethoxysilane (KH550, 99%), CCl4 (98%), and boric acid (H3BO3, 99.5%) were received from Shanghai Macklin Biochemical Co., Ltd. FDTS and Brilliant Green (CAS: 633-03-4) were purchased from Shanghai Aladdin BioChem Technology Co., Ltd. n-Hexane (99.5%) was purchased from Shandong Xiya Chemical Industry Co., Ltd. Sudan I (CAS: 842-07-9) was bought from Sinopharm Chemical Reagent Co., Ltd. TiO2 nanoparticles (CAS: 13463-67-7) were provided by Hangzhou Hengna New Materials Co., Ltd. Sodium hydroxide (NaOH, 96%) was purchased from Hangzhou Gaojing Fine Chemicals Co., Ltd.

Preparation of the TiO2–PVA Sponge

The pristine PVA sponge was first ultrasonically cleaned in an ethanol solution for 30 min and then dried in an oven at 70 °C. Then, 0.12 g TiO2 nanoparticles were added into 100 mL NaOH aqueous solution (pH = 10), followed by mixing and magnetically stirring for 2 h to obtain a suspension solution of TiO2 nanoparticles. Subsequently, 1 g H3BO3 was added into the above suspension solution, and the mixed solution was stirred for another 2 h at 85 °C. After that, 0.5 g KH550 was added to the above mixed solution, and the pre-cleaned PVA sponge was immersed in the solution. The reaction was kept at 85 °C for 3 h under stirring, and the resulting PVA sponge was taken out and dried in an oven at 60 °C. Finally, the resulting PVA sponge was immersed in a solution of FDTS in n-hexane (0.5 wt %) for 30 min and dried at 80 °C for 2 h to obtain a superhydrophobic TiO2–PVA sponge.

Characterization

A scanning electron microscope (Apreo, FEI) was used to analyze the morphologies of the pristine PVA sponge and the as-prepared superhydrophobic/superoleophilic TiO2–PVA sponge. The chemical composition of the TiO2–PVA sponge was measured by EDS and FTIR (Nicolet iS 10, Thermo Fisher). The surface crystal structures of the superhydrophobic TiO2–PVA sponge were characterized by X-ray diffraction (XRD, SMART LAB, Rigaku). The WCA was tested and repeated with a deionized water droplet of ∼5 μL by using a contact angle goniometer (DSA25, KRüSS) at room temperature.[69−72] The size distribution of TiO2 nanoparticle dispersion was characterized by using dynamic light scattering (Nano BT-90).

Stability Measurements

The chemical stability of the TiO2–PVA sponge was tested by immersing the TiO2–PVA sponge in different solutions (i.e., n-hexane, CCl4, and ethanol) for 24 h and different pH solutions (i.e., pH = 1, pH = 7, and pH = 13) for 25 h. To assess the thermal stability, the TiO2–PVA sponge was immersed in boiling water for different times (0–60 min). Furthermore, to evaluate the sustainability of the superhydrophobic/superoleophilic TiO2–PVA sponge, WCAs were tested after sandpaper abrasion tests. The TiO2–PVA sponge was pressed against an 800 mesh sandpaper with a 100 g stainless weight and was then pulled by a pair of tweezers at a speed of 0.1 cm/s. Besides, the durability of the TiO2–PVA sponge was also tested by ultrasonification (Kunshan Ultrasonic Instrument, 150 W). The TiO2–PVA sponge is first ultrasonified for a certain time (i.e., 0–60 min) and then dried in an oven, followed by evaluating the contact angles of a droplet on the TiO2–PVA sponge.

Oil Absorption Experiments

In order to evaluate the absorption capacity of the TiO2–PVA sponge, a piece of the superhydrophobic/superoleophilic TiO2–PVA sponge was immersed in various solvents and oils (i.e., PEG, CCl4, liquid paraffin, ethanol, edible oil, and n-hexane) for 1 min until the sponges were saturated with liquids, and then the weight of the saturated sponges was measured. The absorption capacity (Q) was calculated by the following equationwhere M0 (g) and M1 (g) represent the weight of the TiO2–PVA sponge before and after absorption, respectively. The recyclability of the TiO2–PVA sponge was characterized by the following process. After finishing an absorption process, the TiO2–PVA sponge was washed by anhydrous ethanol and then dried in an oven at 60 °C. The above process was repeated 15 times to evaluate the recyclability of the TiO2–PVA sponge.

Oil/Water Separation Experiment

A self-made device was used for the oil/water separation experiment. The superhydrophobic/superoleophilic TiO2–PVA sponge was first fixed in a funnel. Then, the mixture of water and CCl4 was poured into the funnel to realize the separation of oil from water. In this process, water was repelled by the TiO2–PVA sponge in the funnel, while oil could easily leak through the TiO2–PVA sponge because of its superhydrophobicity and superoleophilicity. The separation efficiency (η) was calculated by the following equationwhere Ma (g) and Mb (g) are the weight of the oil before and after oil/water separation, respectively, and η (%) is the separation efficiency of the TiO2–PVA sponge.

Continuous In Situ Oil/Water Separation

A continuous oil/water separation device was designed by using a vacuum pump system consisting of a Büchne flask and rubber tube. One end of the rubber tube was attached to the superhydrophobic/superoleophilic TiO2–PVA sponge, and the other end was connected to the Büchne flask. In this test, the sponge was immersed in the oil/water interface of the mixture solution, and the oil/water separation was continuously carried out with the aid of a vacuum pump.